Mechanical springs are used in a variety of audio applications. In typical applications, springs are driven by an input audio signal near one end. Depending upon the direction of the driving force, particular energy modes or combinations of modes will be excited on the spring. A delayed signal will then appear at the other end, with the amount of delay being determined by the wave propagation speed for the mode(s) excited. As shown in FIGS. 1A, 1B and 1C, helical springs support at least three transmission modes for mechanical vibrations at audio frequencies, including transverse, longitudinal and torsional modes, respectively.
In one early application dating back to the 1920s, springs were used to delay audio signals for telephone applications such as echo cancellation. See, e.g., R. L. Wegel, “Wave Transmission Device,” U.S. Pat. No. 1,852,795, Apr. 5, 1932. Helical springs have also long been used for artificial reverberation (see, e.g., H. E. Meinema et al., “A New Reverberation Device for High Fidelity Systems,” Journal of the Audio Engineering Society, vol. 9, no. 4, October, 1961 and U.S. Pat. No. 3,938,063) and are still a popular choice for guitar amplifer reverberators.
In the late 1930s, Hammond configured springs so that propagating waves would be reflected back and forth, creating a series of echoes reminiscent of reverberation. See Laurens Hammond, “Electrical Musical Instrument,” U.S. Pat. No. 2,230,836, Feb. 4, 1941. Modern day mechanical spring reverberators take a very similar approach, often using two or three springs of different lengths in parallel to increase echo density, and driving the springs torsionally to minimize susceptibility to mechanical shock. See, e.g., Meinema et al., supra; Alan C. Young, “Artificial Reverberation Unit,” U.S. Pat. No. 3,106,610, Oct. 8, 1963.
While the sound of a mechanical spring reverberator is desirable in a number of settings, its physical form limits its use. A digital emulation of a spring reverberator, on the other hand, would have many advantages over a physical unit, including repeatability and automation of control parameters. To be useful, such an emulation should effectively reproduce the sound of the hardware unit, and should be computationally efficient. However, problems exist that make such a useful emulation difficult to achieve.
For example, it could be argued that springs are approximately linear and time invariant at typical reverberator operating levels, and, as a result, may be characterized by their impulse responses. Accordingly, in one possible approach, a spring reverberator could be emulated by convolving an input signal with its measured impulse response. The difficulty is that such a convolution is expensive, both in computation and memory usage. There remains a need in the art, therefore, to develop a computationally efficient, accurate digital emulation of a spring reverberator.
Another drawback of a theoretical convolutional emulation of a spring reverberator is that it would be inflexible because different impulse responses would be required for each set of spring parameters desired, and it would be expensive to smoothly change from one parameter set to another. There remains a need in the art, therefore, to develop a discrete-time emulation of a spring reverberator which could be parameterized according to spring characteristics, which can be efficiently and smoothly changed.
In some musical genres, such as surf music, it is not unusual to rock, kick, or otherwise disturb the reverberation unit in time with certain musical events. These mechanical disturbances create distinctive sounds, and would preferably be included in a discrete-time spring reverberator emulation. These sounds could be recorded and played back like the sampling and playback of drums or other instruments in sampling synthesizers. However, such a system would require a lot of memory, and would not be easily adapted to varying spring or disturbance parameters. There remains a need in the art, therefore, to provide a spring reverberator which could efficiently and accurately produce the effect of a mechanical disturbance to the spring reverberator unit.